Mechanical testing of materials, including tensile and compression testing, is a well known art. In conventional mechanical testing, a macroscopic sample of a material of interest is prepared and is subjected to mechanical loads under various conditions to determine one or more parameters of interest. Various standards for performing mechanical testing have been described, including for example, ASTM International (formerly American Society for Testing and Materials or ASTM) standards. Conventional macroscopic crystalline samples generally comprise a plurality of sections having one or more of different crystallographic orientations, and different grain structures. Accordingly, a typical macroscopic specimen yields data that includes the effects of grain boundaries, thermal and mechanical treatments that can result in defects such as point defects, edge and screw dislocations, slip, stacking faults, and other defects.
In a tensile test, also known as a tension test, an axial pull is exerted upon the specimen of interest in accordance with agreed upon standards, and the results measured with scientifically accurate methods. Examples of parameters and relationships that can be determined using tensile testing include true stress and strain, engineering stress and strain, the elastic modulus, the ultimate tensile strength, the fracture stress, the modulus of toughness, and the modulus of resilience.
A compression test determines behavior of materials under loads that may be sufficient even to crush the specimen of interest. The specimen is compressed and deformation at various loads is recorded. Commonly, compressive stress and strain are calculated and plotted as a stress-strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and, for some materials, compressive strength.
The ASM Handbook®, Vol. 8, Mechanical Testing and Evaluation, ASM International, Materials Park, Ohio 44073-0002, states: “Axial compression testing is a useful procedure for measuring the plastic flow behavior and ductile fracture limits of a material. Measuring the plastic flow behavior requires frictionless (homogenous compression) test conditions, while measuring ductile fracture limits takes advantage of the barrel formation and controlled stress and strain conditions at the equator of the barreled surface when compression is carried out with friction. Axial compression testing is also useful for measurement of elastic and compressive fracture properties of brittle materials or low-ductility materials. In any case, the use of specimens having large L/D ratios should be avoided to prevent buckling and shearing modes of deformation.”
Hardness testing is conventionally performed using an indenter that is pressed into a surface of a material, and the resulting deformation is examined and quantified. Examples of standard hardness measurements include Rockwell hardness, Vickers hardness, and Brinell hardness.
Up to now, the application of such testing procedures to nano-scale specimens has not been convenient, and to the inventor's knowledge, no one has performed tensile testing on such specimens. In the field of testing of nano-scale specimens of materials of interest, mechanical deformation has largely been carried out in thin films due to their relative ease of deposition and their industrial relevance. Thin films' mechanical properties like the elastic modulus, hardness, and stress-strain can be determined via nanoindentation, which involves indenting a sharp diamond tip into the material and measuring the load as a function of displacement into the surface. In all nanoindentation studies, a so-called size effect is observed, which manifests itself as an increase in hardness at shallower indentation depths. Various groups of scientists and engineers are studying size effects in small specimens by uniaxial compression of nano-pillars, nanotube and nanowire forests. In these experiments, a nanoindenter with a flat tip is used to conduct compression tests rather than nanoindentation tests. This testing capability proves to be useful in any nano-scale fabrication as it provides a reliable way of assessing the mechanical properties of a structure, such as elastic response, yield stress, and possibly fatigue parameters. Although a unified theory explaining plasticity below a certain length scale remains a matter of great research and controversy, the results of most computational and experimental studies indicate that smaller is always stronger. Therefore, it has been determined that mechanical properties of a particular material are different at the nano-scale and cannot be inferred from its bulk properties.
While these nano-compression experiments are effective for determination of some of the mechanical parameters at the nano-scale, they are mainly used by research groups and are not commercially available. Moreover, there is a need to have additional mechanical characterization techniques for nano-scale samples. For example, reliability concerns in MEMS and NEMS fabrication usually require the knowledge of a material's strength, ductility, tensile toughness, and fracture toughness, which most likely differ from those in the bulk. These and other properties can be obtained by performing tension rather than compression experiments. Tension experiments currently present a great experimental challenge and have not been widely performed. It is believed that there are only two in-situ SEM systems capable of compression load-displacement measurements, one at the Wright-Patterson Air Force Base and one at EMPA, an affiliate of the Swiss Federal Institute of Technology. Neither system is equipped with the tensile testing capability at the desired scale, below 1 μm.
There is a need for systems and methods for making tensile (and also compression) tests on nano-scale specimens, in order to determine the fundamental materials properties.